This invention relates generally to the field of power converters. More particularly, the present invention relates, in certain embodiments, to a Buck or Boost power converter providing high conversion efficiency especially suitable for cellular telephones, selective call two way radios and other battery powered devices.
As more people become dependent upon battery powered devices such as radios and cellular telephones, high efficiency in the electronics powering such devices becomes increasingly important. For example, in the case of cellular telephones, size and talk time have become critical features. It is possible to improve the talk time and/or reduce size of a cellular telephone by improving the efficiency of the RF transmitter. The efficiency of the RF transmitter can be substantially increased by using an optimal supply voltage based on the instantaneous envelope of the RF signal being transmitted. Cellular telephones typically use batteries such as lithium ion batteries which range from approximately 4.5 V to 2.75 V output based on the state of charge of the battery. It is therefore highly desirable to have a power converter within such a cellular telephone that provides a voltage both higher and lower than the battery voltage. This permits the radio transmitter circuit to most efficiently transmit the RF signal by adjusting the voltage to the RF power amplifier to track the envelope of the RF signal being transmitted.
Such power conversion can be accomplished utilizing known Buck-Boost converters. Unfortunately, known Buck-Boost converters often use many large magnetic elements and have comparatively poor efficiency (typically approximately 80%). In such known Buck-Boost converters, a switching signal with a controllable duty cycle D drives cascaded Buck and Boost circuits to producer an output voltage which a function of the duty cycle. In addition to comparatively poor efficiency, known Buck-Boost converters typically invert the polarity of the output voltage. Those converters known to not convert the polarity of the output voltage frequently have even lower power conversion efficiency.